The present invention relates to a heat sensitive circuit breaker for electrical apparatus, the circuit breaker being of the type which includes an insulated body having electrical terminals and contact springs to produce an electric current path, a heat transfer plate, an insert of meltable material constituting a thermal actuator, and a force-transmitting pin of insulating material which is displaceably mounted in the insulated portion. One end of the force-transmitting pin is in engagement with the insert and its other end bears against one of the contact springs.
Heat sensitive circuit breakers employing meltable, or fusible, materials are known, one particular form being as disclosed in German Pat. No. 2,012,426, in which an electrically conductive sleeve contains a mass of meltable salt which is connected, via a compression spring, with an electrically conductive component which is displaceable within the sleeve and which produces an electrical connection with an electrical conductor that is otherwise insulated from the sleeve.
When a predetermined temperature is reached, the mass of salt melts, and hence collapses, causing the component to be displaced in the sleeve so as to sever the electrical connection between the sleeve and the electrical conductor.
A drawback of this known structure, however, is that the sleeve carries current and when it is coupled to a heat transfer surface which is usually exposed and hence liable to be touched, it must be electrically insulated in a suitable manner.
This necessary insulation constitutes an additional cost to the user of such heat sensitive circuit breakers. Moreover, differences in heat transfer under different insulation conditions adversely influence the thermal response behavior of such devices.
A further drawback of known devices is the lack of fastening means, since with such heat sensitive circuit breakers fastening can be effected only by means of a clamp for which again the user must provide suitable means.
Other heat sensitive circuit breakers employing meltable materials are known in which contact springs and electrical terminals are disposed in an insulated member and a meltable insert or solder is used as the thermal actuator. A force-transmitting pin of insulating material is displaceably mounted between the solder and the contact springs and acts to separate the contact springs once the solder insert has melted.
In known devices of this type, a solder body in the shape of a circular disc is used as the meltable insert, the disc being placed on a heat transfer plate which has an opening of the size of the force-transmitting pin. When the solder melts, the pin pushes the solder into the opening of the heat transfer plate and the contact springs are separated.
However, due to its poor long-term behavior, this type of device is extremely disadvantageous for use as a heat-sensitive circuit breaker.
Although the solders employed in these devices should have a defined melting point based on their alloy composition, a solder begins to soften before it reaches its melting point. That means that under prolonged exposure to heat at temperatures below the melting point, there occurs a reduction in hardness and a force--for example from the spring tension of the contact spring--can prematurely cause the heat sensitive circuit breaker to be actuated.
Moreover, in known devices there exists the drawback that the solder is exposed. It is known to be possible that various solders experience a shift in their melting point under the influence of oxygen due to oxidation.